1. Field of the Invention
This invention relates to the field of thermomechanical, bleached pulp and paper formed from such pulp. More particularly, this invention relates to the field of thermomechanical, bleached aspen wood pulp having reduced yellowing and increased brightness, and to processes of using such pulp in the fabrication of paper where aluminum based wet end chemicals such as alum, sodium aluminate, poly aluminum chloride, poly aluminum silicate sulfate are used as wet end additives.
2. Prior Art
Thermomechanical pulping processes are known. In these processes, wood chips are steamed for a short period of time after which the steamed chips are introduced into the open lip of a mechanical refiner. In the refiner, the wood chips are ground between rotating grooved metal discs. As the wood mass moves from the center of the refiner to the periphery of the refiner, the wood is broken down into progressively smaller particles and finally into fibers. Frequently, both the heating and refining stages are done under pressure; chemical treatments prior to refining or during the heating stage are further modifications to these pulping systems.
Thermomechanically produced pulps tend to be dark and require bleaching to reach the desired level of brightness. Various types of bleaching agents are used. For example, oxidative bleaching agents such as hydrogen peroxide and reductive bleaching agents such as sodium hydrosulfite (also called sodium dithionite) or formamidine sulfinic acid optionally in the presence of additives such as sodium tripolyphosphate (STPP), trisodium nitrilotriacetate, diethylene amino pentaacetate (DTPA), and ethylenediamine tetra acetate (EDTA) are used.
The bleached pulp is then formed into a paper making stock furnish of the desired consistency to which is added a wide range of chemicals or “wet end additives” to impart or enhance specific sheet properties or to improve the process for formation of the sheet. Wet end additives normally used in paper production are aluminum based additives such as alum, sodium aluminate, poly aluminum silicate sulfate and poly aluminum chloride. These additives are used as aids for retaining the wood fiber on the paper forming wire, to fix additives to the fiber surface and to control pH. However, although of great benefit in the production of paper, these aluminum based additives adversely affect the quality of thermomechanical bleached aspen pulp by causing the pulp to yellow. To offset thermomechanical bleached aspen wood pulp yellowing, dyes are used in the wet end of the paper making machine to give the final sheet a blue shade which in turn makes it look whiter. These dyes adhere to fiber fines, giving them a darker appearance. When these darkened fines are recirculated to the head box, they lower the brightness of the sheet. Optical brightness and bleaching agents are then needed to increase brightness to meet sheet specifications. The processes of this invention obviate one or more disadvantages of the prior art processes.